The invention relates to an electrooptical liquid system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix,
in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix,
which exhibits an electrically switchable transparency which is essentially independent of the polarization of the incident light,
the precursor of the PDLC film of which comprises one or more monomers, oligomers and/or prepolymers and a photoinitiator, and is cured photoradically, and
the liquid crystal mixture of which comprises one or more compounds of the formula I 
in which
The preparation of PDLC (=polymer dispersed liquid crystal) films is described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585. In the so-called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is first homogenously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) both being also methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a xe2x80x9cSwiss cheesexe2x80x9d and a xe2x80x9cpolymer ballxe2x80x9d morphology. In the latter one, the polymer matrix consists of small polymer particles or xe2x80x9cballsxe2x80x9d being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastic deterioration of the electrooptical characteristic line when comparing the virgin and the second run.
According to Yamagishi et al., loc. cit., the Swiss cheese morphology is promoted in case the polymerization reaction runs via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favoured in case the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes.
In PDLC films, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index no, is selected in such a way that it more or less coincides with the refractive index np of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle xcex8=0xc2x0) now sees an optically isotropic medium and appears transparent.
No polarisers are required for operating PDLC systems, as a result of which these systems have high transmission. PDLC systems provided with active matrix addressing have been proposed on the basis of these favourable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
The liquid crystal mixtures used for producing PDLC systems have to meet a wide range of demands. One of the refractive indices of the liquid crystal mixture is selected such that it matches with the refractive index of the polymer matrix. The term matching of refractive indices used here covers not only the case no (resp. another refractive index of the liquid crystal mixture) xcx9cnp, but also the condition no (resp. another refractive index of the liquid crystal mixture) less than np which is sometimes chosen to reduce off-axis haze and enlarges the view angle as is described, for example, in EP 0,409,442.
The liquid crystal mixture preferably has a positive dielectrical anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
Furthermore, the liquid crystal mixture should have a high clearing point, a broad nematic range, no smectic phases down to low temperatures and a high stability and should be distinguished by an optical anisotropy xcex94n and a flow viscosity xcex7 which can be optimized with respect to the particular application, and by a high electrical anisotropy.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC systems. The PIPS, SIPS and TIPS technologies are described in some detail in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427. The PDLC systems described in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427 are based on an epoxy film, while in EP. 0,272,585 acrylate systems are given. The PDLC system of WO 89/06264 is based on multifunctional acrylates and multifunctional thioles, and Y. Hirai et al., SPIE Vol. 1257, Liquid Crystal Displays and Applications, 1990, p.2 describe PDLC systems the precursor of the polymer matrix of which being based on monomers and oligomers. Further suitable matrix materials are described, for example, in U.S. Pat. No. 3,935,337, WO 91/13126 and in further references.
Electrooptical systems containing PDLC films can be addressed passively or actively. Active driving schemes employing an active matrix having nonlinear addressing elements like, for example, TFT transistors integrated with the image point, are especially useful for displays with high information content.
When the PDLC system is addressed by means of an active matrix, a further far reaching criterion is added to the requirements listed so far which must be fulfilled by the cured polymer and the liquid crystal mixture being embedded in microdroplets. This is related to the fact that each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the so-called holding ratio (HR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state and the voltage applied; a process for determining the HR is given, for example, in Rieger, B. et al., Conference Proceeding der Freiburger Arbeitstagung Flxc3xcssigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg 1989. Electrooptical systems having a low or relatively low HR show insufficient contrast.
A further serious problem is often that the liquid crystal mixture has insufficient miscibility with the monomers, oligomers and/or prepolymers of the polymer used for forming the matrix, which limits in particular the use of PIPS technology in microdroplet matrix systems.
A further disadvantage is in particular that the liquid crystal mixture or individual components of the liquid crystal mixture are in many cases distinguished by an excessively high and/or significantly temperature dependent solubility in the cured, matrix-forming polymer. If, for example, the solubility or the temperature-dependence of the solubility of one or several components differs quite significantly from that of the remaining components, it may happen that the physical properties of the mixture and in particular also of the refractive indices ne and no are substantially affected, which disturbs the adjustment of no or ne or another refractive index of the liquid crystal mixture to nM, thus resulting in a deterioration of the optical properties of the system.
The xe2x80x9cbleedingxe2x80x9d described in EP 0,357,234, according to which at least some of the liquid crystal droplets have the tendency, when the matrix film is subjected to mechanical stress, to dissolve with diffusion of the liquid crystal to the film surface or into the matrix, is favoured by a high solubility of the liquid crystal mixture in the cured polymer.
Very important electrooptical parameters of electrooptical systems according to the preamble of claim 1 are the switching voltages and switching times. The threshold voltage Vth is usually defined as the voltage V10,0,20 at which a transmission of 10% is observed at a temperature of 20xc2x0 C. and under a viewing angle xe2x8ax96 of 0xc2x0 while the saturation voltage is the lowest voltage for which the maximum transmission is observed at 20xc2x0 C. and a viewing angle of 0xc2x0. The switching on time ton is usually reported as the time necessary for the transmission to rise from 0% to 90% of the maximum transmission when the saturation voltage is applied while toff is the time necessary for the transmission to drop from 100% to 10% when the voltage is switched off.
In U.S. Pat. No. 4,673,255 it is shown that a correlation exists between the mean size of the microdroplets on the one hand and the switching voltages and switching times of the system on the other hand. Generally, relatively small microdroplets cause relatively high switching voltages, but relatively short switching times and vice versa.
Experimental methods for influencing the average droplet size are described, for example, in U.S. Pat. No. 4,673,255 and in J. L. West, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt., 157, 1988, 427. In U.S. Pat. No. 4,673,255, average drop diameters between 0.1 xcexcm and 8 xcexcm are given, while, for example, a matrix which is based on a glass monolith has pores having a diameter between 15 and 2,000 xc3x85. For the mesh width of the network of PN systems, a preferred range between 0.5 and 2 xcexcm is given in EP 0,313,053.
The switching voltage, however, must not be chosen too high because of several reasons (power consumption, safety of operation, compatibility with conventional modules of microeletronic).
On the other hand, high switching times are generally not tolerable which is evident in case of display applications, but which is also true for many other applications. Low switching time are also often required at lower temperature because the systems according to the preamble are also discussed for out-door applications.
It is true that considerable efforts have already been undertaken hitherto in order to optimize PDLC systems with respect to the liquid crystal mixture used and the polymer system. On the other hand, however, it is still an open problem how to realize PDLC films which are characterized both by low switching times especially at low temperatures and at the same time by advantageous values of the switching voltages. No method is known so far by which switching voltages and switching times can be adjusted with respect to the intended application more or less independently from each other.
Furthermore, only few investigations of PDLC systems having active matrix addressing can be found in the literature, and no concepts have so far been proposed for providing electrooptical systems having
a high HR and a low temperature dependence of HR
advantageous values of the switching voltages, and
low switching times, especially at low temperatures.
Consequently, there is a high demand for PDLC systems which fulfill to a large extent the requirements described and which exhibit advantageous values of the switching voltages, and, in particular, low switching times especially at low temperatures. Furthermore, there is a high demand for actively addressed PDLC systems which exhibit a high HR and a low temperature dependence of HR in addition to low switching times.
The object of the invention was to provide PDLC systems of this type and precursors of these PDLC systems containing monomers, oligomers and/or prepolymers of the polymer used and a liquid crystal mixture. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It has been found that PDLC systems which are characterized by low switching times can be obtained if one or more reactive liquid crystalline compounds are added to its liquid crystal mixture.
The invention thus relates to an electrooptical liquid crystal system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix,
in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix,
which exhibits an electrically switchable transparency which is essentially independent of the polarization of the incident light,
the precursor of the PDLC film of which comprises one or more monomers, oligomers and/or prepolymers and a photoinitiator, and is cured photoradically, and
the liquid crystal mixture of which comprises one or more compounds of the formula I 
in which
characterized in that the liquid crystal mixture additionally contains one or more reactive liquid crystalline compounds in order to obtain improved switching times especially at low temperatures. Part of the reactive liquid crystalline compounds which can be used in the electrooptical systems according to the present invention is new, and such new reactive liquid crystalline compounds are also claimed.
Specifically, the present invention also relates to reactive liquid crystalline compounds of formula III
R1xe2x80x94Pxe2x80x94Xxe2x80x94A3xe2x80x94Zxe2x80x94A4xe2x80x94R2xe2x80x83xe2x80x83III
wherein R1 is CH2xe2x95x90CWxe2x80x94COOxe2x80x94, CH2xe2x95x90CHxe2x80x94, 
xe2x80x83HWNxe2x80x94, HSxe2x80x94CH2xe2x80x94(CH2)mxe2x80x94COOxe2x80x94 with W being H, Cl or alkyl with 1-5 C atoms and m being 1-7,
P is alkylene with up to 12 C atoms, it being also possible for one or more CH2 groups to be replaced by O,
X is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or a single bond,
R2 is alkyl radical with up to 15 C atoms which is unsubstituted, mono- or polysubstituted by halogen, it being also possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a manner that oxygen atoms are not linked directly to one another, xe2x80x94CN, xe2x80x94F, xe2x80x94Cl, or alternatively R2 xe2x80x94Pxe2x80x94X,
A3 is a 1,4-phenylene or a napthalene-2,6-diyl radical which is unsubstituted or substituted with 1 to 4 halogen atoms,
A4 is 
xe2x80x83it being possible for radicals (a) and (b) to be substituted by CN or halogen and one of the 1,4-phenylene groups in (a) and (b) can also be replaced by a 1,4-phenylene radical in which one or two CH groups are replaced by N, and
Z is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond.
The construction of the electrooptical system according to the present invention corresponds to the customary mode of construction for systems of this type. The term customary mode of construction is in this case broadly interpreted and includes all adaptations and modifications.
Thus, for example, the matrix formed by the transparent medium in which the liquid crystal mixture is microdispersed or microencapsulated, is arranged between conducting electrodes like a sandwich.
The electrodes are applied, inter alia, to substrate sheets of, for example, glass, plastic or the like; if desired, however, the matrix can also be provided directly with electrodes so that the use of substrates can be avoided. One of the electrodes forms an active matrix while the other one acts as counter electrode.
The precursor of the PDLC film comprising the precursor of the matrix, the liquid crystal mixture and one or more reactive liquid crystalline compounds can be capillary filled between two substrates which are provided with electrode layers, and the precursor of the PDLC film is subsequently cured, for example, by irradiation with UV light. Another technique comprises coating of the precursor of the PDLC film on a substrate with subsequent curing. The film may be peeled off and arranged between 2 substrates provided with electrode layers. It is also possible that the substrate onto which the precursor of the PDLC film is applied exhibits an electrode layer so that the electrooptical system can be obtained by applying a second electrode layer and, optionally, a second substrate onto the coated and cured film.
The electrooptical system according to the invention can be operated reflectively or tramsmissively so that at least one electrode and, if present, the associated substrate are transparent. Both systems customarily contain no polarizers, as a result of which a distinctly higher light transmission results. Furthermore, no orientation layers are necessary, which is a considerable technological simplification in the production of these systems compared with conventional liquid crystal systems such as, for example, TN or STN cells.
Processes for the production of PDLC films are described, for example, in U.S. Pat. Nos. 4,688,900, 4,673,255, 4,671,618, WO 85/0426, U.S. Pat. No. 4,435,047, EP 0,272,595, Mol. Cryst. Liq. Cryst, Inc. Nonlin. Opt. 157 (1988) 427, Liquid Crystals, 3 (1988) 1543, EP 0,165,063, EP 0,345,029, EP 0,357,234 and EP 0,205,261. The formation of the PDLC film is generally achieved by 3 basic methods: in the PIPS technique (=PIPS, polymerization induced phase separation) the liquid crystal mixture, and optionally further additives, are dissolved in the precursor of the matrix material, and subsequently polymerization is started. TIPS (=thermally induced phase separation) means that the liquid crystal mixture is dissolved in the melt of the polymer followed by cooling while SIPS (=solvent induced phase separation) starts with dissolving the polymer and the liquid crystal mixture in a solvent with subsequent evaporation of the solvent. The invention is, however, not restricted to these specific techniques but covers also electrooptical systems obtained by modified methods or other methods. The use of the PIPS technology is usually preferred.
The thickness d of the electrooptical system is customarily chosen to be small in order to achieve a threshold voltage Vth which is as low as possible. Thus, for example, layer thicknesses of 0.8 and 1.6 mm are reported in U.S. Pat. No. 4,435,047, while values for the layer thickness between 10 and 300 xcexcm are given in U.S. Pat. No. 4,688,900 and between 5 and 30 xcexcm in EP 0,313,053. The electrooptical systems according to the invention only have layer thicknesses d greater than a few mm in exceptional cases; layer thicknesses below 200 xcexcm and especially below 100 xcexcm are preferred. In particular, the layer thickness is between 2 and 100 xcexcm, especially between 3 and 50 xcexcm and very particularly between 3 and 25 xcexcm.
An essential difference between the electrooptical liquid crystal system according to the present invention and those customary hitherto, however, consists in that the liquid crystal mixture contains one or more reactive liquid crystalline compounds.
The term reactive liquid crystalline compounds denotes rod-like compounds of formula II
Rxe2x80x2xe2x80x94Gxe2x80x2xe2x80x94Rxe2x80x3xe2x80x83xe2x80x83II
wherein at least one of the terminal groups Rxe2x80x2 and Rxe2x80x3 is a reactive group exhibiting one reaction site such as a hydroxyl group HOWxe2x80x22Cxe2x80x94, a thio group HSWxe2x80x22Cxe2x80x94, an amino group HWxe2x80x2Nxe2x80x94, a carboxyl group, an epoxide group 
or an isocyanate group Oxe2x95x90Cxe2x80x94Nxe2x80x94, or a polymerizable reactive group exhibiting two or more reactive sites such as a vinyl type group Wxe2x80x22Cxe2x95x90CWxe2x80x2xe2x80x94, a (meth)acrylate type group 
a styrene type group 
with Wxe2x80x2 being independently from each other H or an alkyl group with 1-5 C atoms, the other terminal group is also, independently from the first terminal group, a reactive group with one or more reactive sites or an alkyl radical with up to 15 C atoms which is unsubstituted or mono- or polysubstituted by halogene, it being also possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a manner that O atoms are not linked directly to one another,
Gxe2x80x2 is a rod-like diyl group of the formula
xe2x80x94S1xe2x80x94(A5xe2x80x94Z3)mxe2x80x94A6xe2x80x94S2xe2x80x94
with S1 and S2 being independently from each other alkylene groups with 0-20 atoms which can be linear or branched, it also being possible for one or more CH2 groups to be replaced, in each case independently from each other, by xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NWxe2x80x2xe2x80x94 with the proviso that O atoms are not linked directly to one another,
A5 and A6 denote, independently from each other,
a) a cyclohexylene group, wherein one or two non-adjacent CH2 groups may be replaced by O or S atoms,
b) an unsubstituted 1,4-phenylene group wherein one to three CH groups may be replaced by xe2x80x94Nxe2x80x94 or a 1,4-phenylene group which is mono- or polysubstituted by F, Cl and/or CH3,
c) a bicyclo(2,2,2)octylene group, a naphthalene-2,6-diyl group, a decahydronaphthalene-2,6-diyl group or 1,2,3,4-tetrahydronaphthalene group,
Z3 is independently from each other xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CCxe2x80x94 or a single bond, and
m denotes 1,2,3, or 4.
Above and below, the term reactive liquid crystalline compounds refers to reactive rod-like molecules like, for example, those of formula III or other rod-like reactive compounds which may be enantiotropic, monotropic or isotropic, preferably, however, enantiotropic or monotropic.
In a preferred embodiment of the eletrooptical systems according to the present invention, at least one of Rxe2x80x2 and Rxe2x80x3 preferably is or contains an ene-group 
When polymerizing the precursor of the PDLC film by impact of thermal energy or irradiation, usually in presence of an ionic or radical polymerization initiator, the reactive liquid crystalline compounds being contained in the liquid crystalline phase when phase separation starts, are reacted with each other thus obviously forming some internal structure in the liquid crystalline microdroplets. This structure may be considered as some kind of network which divides the liquid crystalline microdroplet in some smaller sub-compartments which may be in contact with each of be separated from each other. The term xe2x80x9csome kind of networkxe2x80x9d is to be understood in a wide sense and comprises a wide range of geometries of the internal structure. The surrounding polymer matrix and the internal structure may be connected or not.
In another embodiment of the electrooptical systems according to the present invention, at least one of Rxe2x80x2 and Rxe2x80x3 is a reactive group exhibiting one reactive site, and in particular a hydroxyl group, a thiol group, a carboxyl group, an amino group or an isocyanato group. Reactive liquid crystalline compounds of this type can be attached to the surrounding polymeric matrix in a coupling reaction or they can also react with each other, especially in case of suitably chosen co-reactive compounds of formula II. The coupling reaction may occur during the polymerization of the surrounding matrix or afterwards as a polymer-analogous reaction. In case of reactive liquid crystalline compounds of formula II exhibiting only one reactive group of the one reaction site type, it is assumed that the reactive group is coupled to the inner surface of the polymeric matrix with the rest of the molecule being arranged in the liquid crystalline microdroplet, inducing there same kind of internal structure.
The addition of one or more reactive liquid crystalline compounds of formula II exhibiting two reactive groups Rxe2x80x2 and Rxe2x80x3 to the liquid crystalline mixture is generally preferred. Also preferred is the addition of a reactive liquid crystalline component, containing at least two different reactive liquid crystalline compounds according to formula II at least one of which contains 2 reactive groups Rxe2x80x2 and Rxe2x80x3. Reactive liquid crystalline components containing at least one reactive liquid crystalline compound with one reactive group Rxe2x80x2 (monofunctional reactive liquid crystalline compound) and at least one reactive liquid crystalline compound with two reactive compounds (difunctional reactive liquid crystalline compound) often are especially preferred while reactive liquid crystalline components consisting of one or more monofunctional reactive liquid crystalline compounds usually are less advantageous.
Especially preferred difunctional reactive liquid crystalline compounds are di-ene type compounds such as divinyls, diacrylates or dimethacrylates, furthermore diols, dithiols and diisocyanates, but also compounds with different reactive groups such as ene-ols, ene-thiols, vinylacrylates etc.
The groups S1 and S2 acting as spacer groups between the reactive groups Rxe2x80x2 and Rxe2x80x3 and the mesogenic core xe2x80x94(A5xe2x80x94Z3)mxe2x80x94A6 are independently from each other an alkylene group with 0-20 C atoms which can be linear or branched, it also being possible for one or more CH2 groups to be replaced, in each case independently from each other by xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NWxe2x80x2xe2x80x94 with the proviso that oxygen atoms are not linked directly to one another.
The length and the structure of the groups S1 and S2 determine whether the mesogenic group exhibits a more or less pronounced degree of flexibility. The following list of suitable groups S1 and S2 is intended to be illustrative and not limiting:
ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, decylene, undecylene, dodecylene, octadenylene, ethyleneoxyethylene, methyleneoxybutylene, ethylenethioethylene, ethylene-N-methyliminoethylene, (1-oxy)methyleneoyloxy, (2-oxy)ethyleneoyloxy, (3-oxy)propyleneoyloxy, (4-oxy)butyleneoyloxy, (5-oxy)pentyleneoyloxy, (6-oxy)hexyleneoyloxy, (7-oxy)heptyleneoyloxy, (8-oxy)octyleneoyloxy, (1-oxy)methyleneoxycarbonyl (2-oxy)ethyleneoxycarbonyl, (3-oxy)propyleneoxycarbonyl, (4-oxy)butyleneoxycarbonyl, (5-oxy)pentyleneoxycarbonyl, (6-oxy)hexyleneoxycarbonyl, (7-oxy)heptyleneoxycarbonyl and (8-oxy)octyleneoxycarbonyl.
The mesogenic core xe2x80x94(A5xe2x80x94Z3)mxe2x80x94A6 of the reactive liquid crystalline compounds can exhibit 2, 3, 4 or 5 rings:
xe2x80x94A5xe2x80x94Z3xe2x80x94A6xe2x80x94xe2x80x83xe2x80x83(1)
xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A6xe2x80x94xe2x80x83xe2x80x83(2)
xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A6xe2x80x94xe2x80x83xe2x80x83(3)
xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A5xe2x80x94Z3xe2x80x94A6xe2x80x94xe2x80x83xe2x80x83(4)
Especially preferred for use in the electrooptical systems according to the present invention are reactive liquid crystalline compounds exhibiting 2-, 3- or 4-ring mesogenic groups according to formula (1)-(3) and in particular 2- or 3-ring mesogenic groups according to formula (1) or (2).
In the following, for sake of simplicity, Cyc is a 1,4-cyclohexylene group, Phe is a 1,4-phenylene group which can be unsubstituted or mono-, di- or trifluorinated, Dio is a 1,3-dioxane-2,5-diyl group, Pyd is a pyridine-2,5-diyl group, Pyr is a pyrimidine-2,5-diyl group, Pip is a piperidine-1,4-diyl group, Bio is a 1,4-bicyclo(2,2,2)octylene group, Nap is a naphthaline-2,6-diyl group and Thn is a 1,2,3,4-tetrahydronaphthaline-2,6-diyl group; the abbreviations Dio, Pyd, Pyr and Pip comprise all possible positional isomers.
Especially preferred is the following smaller group of mesogenic cores according to formula (2):
-Phe-Z3-Phe-xe2x80x83xe2x80x83(2)a
-Cyc-Z3-Cyc-xe2x80x83xe2x80x83(2)b
-Phe-Z3-Cyc-xe2x80x83xe2x80x83(2)c
-Pyr-Z3-Phe-xe2x80x83xe2x80x83(2)d
-Pyd-Z3-Phe-xe2x80x83xe2x80x83(2)e
-Dio-Z3-Cyc-xe2x80x83xe2x80x83(2)f
In the structures according to formulae (2)a-(2)f Z3 preferably is xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond. Electrooptical systems according to the present invention containing one or more reactive liquid crystalline compounds containing a two-ring mesogenic structure according to formulae (2)a-(2)c generally exhibit especially advantageous properties.
Especially preferred is also the use of reactive liquid crystalline compounds according to formulae II which contain a mesogenic group with 3 rings according to formulae (3)a-(3)f:
-Phe-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(3)a
-Cyc-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(3)b
-Cyc-Z3-Cyc-Z3-Phe-xe2x80x83xe2x80x83(3)c
xe2x80x83-Cyc-Z3-Cyc-Z3-Cyc-xe2x80x83xe2x80x83(3)d
-Pyr-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(3)e
-Pyd-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(3)f
Electrooptical systems containing both at least one 2-ring reactive liquid crystalline compound with a mesogenic group according to formula 2(a)-2(f) and at least one 3-ring reactive liquid crystalline compound with a mesogenic group according to formulae 3(a)-3(f) are preferred.
In the mesogenic structures of formulae (3)a-(3)f Z3 preferably is independently from each other a single bond, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94CH2CH2xe2x80x94. Especially preferred are the following combinations with xe2x80x94representing a single bond:
Electrooptical systems containing one or more reactive liquid crystalline compounds according to formula II which contain a mesogenic group with 4 rings according to formulae (4)a-(4)f exhibit advantageous properties:
xe2x80x83-Cyc-Z3-Phe-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(4)a
-Cyc-Z3-Cyc-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(4)b
-Cyc-Z3-Cyc-Z3-Cyc-Z3-Phe-xe2x80x83xe2x80x83(4)c
-Cyc-Z3-Phe-Z3-Phe-Z3-Cyc-xe2x80x83xe2x80x83(4)d
-Phe-Z3-Phe-Z3-Phe-Z3-Phe-xe2x80x83xe2x80x83(4)e
-Cyc-Z3-Cyc-Z3-Cyc-Z3-Cyc-xe2x80x83xe2x80x83(4)f
In the structures according to formula (4)a-(4)f at least one of Z3 preferably is a single bond. The other two linking groups preferably denote independently from each other a single bond, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 OR xe2x80x94CH2CH2xe2x80x94.
Reactive liquid crystalline compounds have hitherto been known. EP 0,261,712, for example, describes liquid crystalline diacrylates of the formula 
wherein R is a hydrogen atom or a methyl group, Zxe2x80x2 is independently from each other xe2x80x94COOxe2x80x94 or xe2x80x94OCOxe2x80x94(xe2x89xa1xe2x80x94OOC), and B is a flexible spacer, chosen from the group consisting of xe2x80x94(CH2)xxe2x80x94, xe2x80x94(CH2)xxe2x80x94Oxe2x80x94, xe2x80x94(Si(CH3)2xe2x80x94O)xxe2x80x94 wherein x=1-5 or xe2x80x94(CH2xe2x80x94CH2xe2x80x94O)yxe2x80x94O wherein y=1-8, for use in orientation layers of LCDs.
Hikmet describes in Mol. Cryst. Liq. Cryst., 198, 357-70 anisotropic gels which were obtained by curing a mixture of a low-molecular weight liquid crystal and liquid crystalline diacrylates.
The use of reactive liquid crystalline compounds in PDLC systems, however, is not reported in literature and it was completely surprising that PDLC systems the liquid crystalline mixture of which additionally contains one or more reactive liquid crystalline compounds, exhibits short switching times even at low temperature and simultaneously advantageous values of the switching voltages.
In the following table 1, the electrooptical properties of systems according to the invention are compared with the properties of a conventional PDLC system (comparative example 1) resp. with the properties of PDLC systems containing non liquid-crystalline reactive monomers. NOA 65 (prepared by Norland Products) is used as the precursor of the matrix, and E7 from Merck Ltd., GB, which consists of
51.0% of 4-pentyl-4xe2x80x2-cyanobiphenyl
25.0% of 4-heptyl-4xe2x80x2-cyanobiphenyl
16.0% of 4-octoxy-4xe2x80x2-cyanobiphenyl
8.0% of 4-pentyl-4xe2x80x3-cyanoterphenyl
is used as liquid crystalline mixture. The additives used in the respective experiments, and their amount with respect to the mass of the precursor of the PDLC film are given in table 1. The systems in each case are prepared by mixing and optionally heating the constituents of the precursor of the PDLC film to form a clear solution which subsequently is capillary filled together with spacers between 2 glass substrates which are provided with electrode layers. The system is then irradiated with light of suitable wavelength in order to cure the precursor; NOA 65 the composition of which is given in Molecular Crystals Liquid Crystals, 196 (1991), 89-102, contains benzophenone as a photoinitiator. The response time xcfx84 given in table 1 which is the sum of switching on and switching off times, is measured at a drive voltage of 1.5xc3x97Vsat with Vsat being the lowest voltage for which maximum transmission is observed.
It is evident from table 1 that the addition of non-liquid crystalline reactive compounds to the precursor of the PDLC film does not affect the electrooptical properties of the cured PDLC film very much (comparative experiments no. 2 and no. 3). Both the saturation voltage and the switching times are comparable to the values obtained for a conventional system without any reactive additives (comparative experiment no. 1). The reason most presumably is that the non-liquid crystalline reactive additives are incorporated into the polymer matrix and do not give rise to an internal structure of the liquid crystalline microdroplets.
Contrary to this, experiments no. 1-4 show that a drastical reduction of switching times is obtained in case a reactive liquid crystalline compound is added to the precursor of the PDLC film. Especially pronounced is the reduction of switching time at the lower temperature of 0xc2x0 C. While the conventional PDLC system of comparative experiment no. 1 exhibits a switching time xcfx84 (0xc2x0 C.)=283 ms, the switching times of the systems according to the invention as prepared in experiments no. 2-4 exhibit switching times at 0xc2x0 C. between 10 and 47 ms.
When comparing experiments no. 2-4 it can be concluded that the addition of a diacrylate component has contrary effects with respect to switching times and switching voltages. If the concentration of the diacrylate-compound 
is chosen to be 2% with respect to the mass of the precursor of th PDLC film, the switching times both at 20xc2x0 C. and 0xc2x0 C. are very low while the saturation voltage is relatively high and distinctly higher than the saturation voltage of the conventional system according to comparative experiment no. 1.
Reducing the concentration of the diacrylate compound as low as 0.1% gives a saturation voltage of 28 V which is comparable to the saturation voltage of the conventional system according to comparative experiment no. 1, but a distinctly lower switching time especially at 0xc2x0 C. The conditions of preparation are the same in all experiments listed in table 1 (mixing temperature of the precursor of the PDLC matrix, cooling rate, etc.) so that the distribution of microdroplet diameters can be assumed to be more or less the same.
Table 2 summarizes the electrooptical properties of systems each of them containing only one monofunctional reactive liquid crystalline compound. It can be taken from table 2 that the addition of monofunctional reactive liquid crystalline compounds alone is often less advantageous. Both in experiment no. 5 and no. 6 the switching times at least of 0xc2x0 C. are inferior to the switching times of the conventional PDLC system according to comparative experiment No. 1. Especially disadvantageous is often the addition of monofunctional reactive liquid crystalline compounds wherein the non-reactive terminal group is a nitrile group. The use of monofunctional reactive liquid crystalline compounds with a less polar or unpolar non-reactive terminal group such as F, Cl, CF3, OCF3, OCHF2, alkyl or alkoxy, however, and/or the use of reactive liquid crystalline components containing at least one difunctional and at least one monofunctional liquid crystalline compound, is often preferred.
Based on the experiments summarized in table 1 and 2 as well as on further extensive experimental studies, the present inventors have developed the following ideas in order to explain the effects observed when adding reactive liquid crystalline compounds to the precursor of the PDLC film:
The reactive liquid crystalline compounds which are completely soluble (i.e. soluble at any concentration ratio of liquid crystal mixture and reactive additive) or at least highly soluble in the liquid crystal mixture, are polymerized and form a network or some other kind of structure within the droplets. The switching times are the lower the more close-meshed the substructure is. The reactive liquid crystalline compound binds into the interface of polymeric matrix and liquid crystal microdroplet which results in increased anchoring and hence restoring forces on the components of the liquid crystal mixture. This leads to an increase of the switching voltages which is the more pronounced the higher the concentration of the reactive liquid crystalline compounds is. The concentration of the reactive liquid crystalline component has therefore to be adjusted properly in order to realize a drastical reduction of switching times in connection with no or only a tolerable increase in switching voltages.
The explanation outlined is to be understood as hypothesis which does not restrict the present invention.
In extensive experiments it was found out that the concentration of the reactive liquid crystalline component which consists of one or more reactive liquid crystalline compounds, must not be chosen too high and preferably is not more than 5% and especially less than 2.5% with respect to the mass of the precursor of the PDLC film. Particularly preferred are electrooptical systems according to the present invention the reactive liquid crystalline component of which amounts to not more than 1%.
The reactive liquid crystalline compounds can be chosen from the great pool of known and new reactive liquid crystalline compounds embraced by formula II. The reactive liquid crystalline compounds preferably exhibit a high or very high solubility in the liquid crystal mixture.
The reactive liquid crystalline component preferably contains not more than 10 and in particular not more than 5 reactive crystalline compounds. Difunctional reactive liquid crystalline compounds are generally preferred and in case of these compounds, the reactive liquid crystalline component perferably contains 1-6, especially 1-3 and in particular not more than 2 reactive liquid crystalline compounds. Further preferred are reactive liquid crystalline components comprising at least one difunctional and one monofunctional reactive liquid crystalline compound. Further preferred are reactive liquid crystalline components comprising at least one monofunctional reactive liquid crystalline compound with the second terminal group being F, Cl, CF3, OCF3, OCHF2 or non-polar group such as alkyl or alkoxy.
The present inventors further observed that electroopticl systems according to the present invention are characterized by advantageous electrooptical properties and that they exhibits, in particular, no or only very little memory effect.